Electroluminescent device using azomethine-lithium-complex as electron injection layer

ABSTRACT

In OLEDs, improved efficiency is obtained by compounds which can form inter alia electron injection layers of the formula (I) wherein R1 is a 1-5 ring aryl (including polycyclic), aralkyl or heteroaryl group which is optionally substituted with one or more C1-C4 alkyl, alkoxy or cyano; R2 and R3 together form a 1-5 ring aryl (including polycyclic), aralkyl or heteroaryl group which is optionally substituted with C1-C4 alkyl, alkoxy or cyano; R4 is hydrogen, C1-C4 alkyl or aryl; and Ar is monocyclic, bicyclic or tricyclic aryl or heteroaryl which is optionally substituted with one or more C1-C4-alkyl or alkoxy groups, or an oligomer thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 13/458,030, filedon Apr. 27, 2012, which is a division of application Ser. No.12/521,334, filed on Jun. 26, 2009 which is incorporated by reference.Application Ser. No. 12/521,334 is a national stage application ofApplication PCT/GB2007/050769 which claims priority for Application0625865.1 filed on Dec. 29, 2006 in the United Kingdom.

FIELD OF THE INVENTION

This invention relates to novel compounds, comprising said compounds andone or more dopants, and to their use in electro-optical oropto-electronic devices, inter alia optical light emitting devices, forexample in an electron injection layer.

BACKGROUND TO THE INVENTION

Hung et al., “Recent progress of molecular organic electroluminescentmaterials and devices”, Materials Science and Engineering, R 39 (2002),143-222 disclose that bilayer cathodes for OLEDs based e.g. on a thin(0.1-1.0 nm) LiF layer between an aluminium cathode and an aluminiumquinolate electron transport layer exhibit significantly improved I-Vcharacteristics and EL efficiencies. They explain that in OLEDs, themajority carriers are holes owing to their higher mobility and smallerinjection barrier. Therefore, lowering the barrier height to electroninjection is especially important as it leads to a better balance ofelectron and hole currents and results in a dramatic increase inluminance at a fixed bias voltage. The replacement of LiF with CsF oralkaline earth fluorides is also discussed.

U.S. Pat. No. 6,885,149 (Parthasarathy et al., Princeton University)discloses that during fabrication of an OLED, an organic electroninjection layer may be doped with a metal either by depositing anorganic electron injection layer on an ultra-thin layer of lithium or bydepositing an ultra-thin layer of lithium on an organic electroninjection layer, the organic material being e.g.2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP or bathocuproine).Use of a metal doped electron injection layer is also disclosed in U.S.Pat. No. 7,114,638, the organic component of said layer being e.g. thecompound shown below:

US-A-20060040139 (Herron et al., Du Pont) discloses the use of metalSchiff base complexes in double heterostructure OLEDs as host materialin the electroluminescent layer or in the electron transport layer. Thecomplexes are based on aluminium, scandium, yttrium or a rare earthmetal and the Schiff base ligand is bivalent and is e.g. of formula:

However, the use of metal Schiff base complexes as electron injectionmaterials for OLEDs is neither disclosed nor suggested by Herron et al.

SUMMARY OF THE INVENTION

A problem with which invention is concerned is to provide OLEDs ofimproved performance. A further problem with which the invention isconcerned is to provide organic materials useful as OLED layer materialsor components e.g. as electron injection materials.

In one aspect, the invention provides a compound of the formula

wherein

R₁ is a 1-5 ring aryl (including polycyclic), aralkyl or heteroarylgroup which may be substituted with one or more C₁-C₄ alkyl (e.g.methyl), alkoxy (e.g. methoxy) or cyano;

R₂ and R₃ together form a 1-5 ring aryl (including polycyclic), aralkylor heteroaryl group which may be substituted with C₁-C₄ alkyl (e.g.methyl), alkoxy or cyano;

R₄ is hydrogen, C₁-C₄ alkyl (e.g. methyl), aryl (e.g. phenyl ornaphthyl) or heteroaryl; and

Ar is monocyclic, bicyclic or tricyclic aryl or heteroaryl which may besubstituted with one or more C₁-C₄-alkyl or alkoxy groups, or anoligomer thereof.

Compounds of the above formula in which R₄ is hydrogen may be made byreacting a primary aromatic or heteroaromatic amine with an aromatic orheteroaromatic aldehyde to form a Schiff base, followed by reaction ofthe Schiff base with a lithium compound e.g. a lithium alkoxide e.g.lithium t-butoxide. Compounds of the above formula in which R₄ is alkyl,aryl or heteroaryl may be made similarly starting from a secondaryaromatic or heteroaromatic amine.

The compounds of the above formula have the advantage that they givecomparable performance to LiF when used as an electron injection layer,or in some embodiments better performance, but do not require the severedeposition conditions associated with LiF. Embodiments of the compoundscan be handled in air and can be deposited by vacuum sublimation attemperatures significantly below that required for LiF (FIG. 13) e.g.below about 250° C. In addition to vacuum sublimation, embodiments ofthe compounds can be deposited by the organic vapour phase deposition(OVPD) process described inter alia by Universal Display Corporation inwhich organic films are deposited using an inert carrier gas to transferfilms of organic material onto a cooled substrate in a hot-walled,low-pressure (typically 0.1-1 Torr) chamber. The process is stated toachieve relatively high deposition rates compared to vacuum sublimation,to permit better shadow mask patterning for forming arrays of pixels andto be useful for substrates of relatively large size. They can also bedeposited by organo vapour jet printing (OVJP) which produces acollimated vapour jet of organic material and carrier gas and impingesthe jet onto a cooled substrate to form a well defined deposit e.g. ofindividual pixels, see Shtein at al., Direct Mask-Free Patterning OfMolecular Organic Semiconductors Using Organic Vapor Jet Printing, J.Appl. Phys., 2004, 96 (8), 4500 and WO 2005/043641 (Shtein et al.).Embodiments of the above compounds can be dissolved in organic solventsand deposited e.g. to form layers or pixels by solution coating e.g.spin coating or by ink jet printing (see WO 03/067679 and Bharathan etal., Polymer electroluminescent devices processed by inkjet printing,Appl Phys. Lett., (1998) 72 (21), 2660) the contents of which areincorporated herein by reference) the good solution processingproperties being a significant advantage e.g. in the manufacture ofpolymer OLEDs, devices based on conductive polymers e.g. PEDOT. Ink jetprinting has been reported using simple organic solvents e.g. an alcoholor chloroform. Preferred ink jet printing is using the piezo variant,and suitable solvents are dichloroethylene, trichloroethylene, xylenes,N-methylpyrrolidones, dioxane and other high boiling ethers,dichlorobenzene and polyhydroxy compounds. Similar solvents can be usedin spin coating. In some embodiments, the compounds described above maywhen used as electron injection layers give rise to reduced voltagedrift compared to the use of inorganic electron injectors such as LiFand increased device lifetime.

The compounds whose formulae are set out above are believed from MSmeasurements to be capable of forming cluster compounds or oligomers inwhich 2-8 molecules of formula as set out above are associated e.g. inthe form of trimeric, tetrameric, hexameric or octomeric oligomers.Although the invention is not dependent on the correctness of thistheory, it is believed that compounds of the invention may in someembodiments associate in trimeric units having a core structure whichhas alternating Li and O atoms in a 6-membered ring, and that thesetrimeric units may further associate in pairs. The existence of suchstructures in lithium quinolate has been detected by crystallography,see Begley et al., Hexakis(μ-quinolin-8-olato)hexylithium (1): acentrosymmetric doubly stacked trimer, Acta Cryst. (2006), E62,m1200-m1202, the disclosure of which is incorporated herein byreference. Again although the invention is not dependent on thecorrectness of this theory, it is believed that formation of oligomericstructures of this type imparts a greater covalent character to the Li—Obonds which may be responsible for the volatility of many of thecompounds of the invention which enables them to be deposited atrelatively low temperatures by vacuum sublimation. However, otherstructures may also be possible e.g. cubic structures.

In a further aspect, the invention provides an electro-optical oropto-electronic device having a layer comprising a compound as definedabove. Such devices include OLEDs and also e.g. organicphototransistors, organic photovoltaic cells, organic photodetectors,electronic storage devices based on bistable organic molecules andphotoconductive imaging members for creating electrostatic latentimages.

In a yet further aspect the invention provides an optical light emittingdiode device having a first electrode, a layer comprising a compound asset out above and a second electrode. The layer is in an embodimentlocated on the cathode and is an electron injection layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated in the accompanyingdrawings in which:

FIGS. 1-12 are graphs showing performance data for OLED devices ofindicated structure; and

FIG. 13 is a graph showing rate of vacuum sublimation as a function oftemperature for compounds of the invention and for lithium fluoride.

DESCRIPTION OF PREFERRED FEATURES Doped Materials

Compounds whose formulae are as set out above may be doped with a rangeof materials for a range of purposes.

Where they are to serve as electron injection layers they may be dopedwith low work function metals e.g. Li, Cs, K, Ca, Ba or complexesthereof e.g. by exposure of the compound in vacuo to vapour of the metalwith which the compound is desired to be doped. For exampleUS-A-2006/0079004 (Werner et al, the disclosure of which is incorporatedherein by reference) explains that Cs is commonly used because Cs dopedorganic semiconductors exhibit relatively high stability. Doping byexposure of the organic semiconductor to Cs can be carried out atmoderate temperatures about 300° C. using a GaCs alloy e.g. Ga₇Cs₁₁.They may also be mixed or doped with complexes e.g quinolates.

Compounds whose formulae are as set out above may be mixed with electrontransport materials. Kulkarni et al., Chem. Mater. 2004, 16, 4556-4573(the contents of which are incorporated herein by reference) havereviewed the literature concerning electron transport materials (ETMs)used to enhance the performance of organic light-emitting diodes(OLEDs). In addition to a large number of organic materials with whichthe present compounds can be mixed they discuss metal chelates, withwhich the present compounds may additionally or alternatively be mixedincluding aluminium quinolate, which they explain remains the mostwidely studied metal chelate owing to its superior properties such ashigh EA (˜−3.0 eV; measured by the present applicants as −2.9 eV) and IP(˜−5.95 eV; measured by the present applicants as about −5.7 eV), goodthermal stability (Tg˜172° C.) and ready deposition of pinhole-free thinfilms by vacuum evaporation. Aluminium quinolate remains a preferredmaterial both for use as a host to be doped with various fluorescentmaterials to provide an electroluminescent layer and for use as anelectron transport layer. More recently zirconium and hafnium quinolateshave been disclosed as electron transport materials, seePCT/GB2007/050737 (Kathirgamanathan et al.) the contents of which areincorporated herein by reference, and the compounds whose formulae areset out above may also be mixed with zirconium or hafnium quinolate.There may also be used e.g. azole compounds such as2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD) and3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (TAZ);phenanthrolines such as 4,7-diphenyl-1,10-phenanthroline (DPA) and2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (DDPA); and mixturesthereof.

When incorporated into electroluminescent layers, a compound of theformula set out above may serve as a host material and may be mixed ordoped with a fluorescent material or with a phosphorescent material.Such materials are reviewed below in relation to the electroluminescentlayer.

Cell Structure

The OLEDs of the invention are useful inter alia in flat panel displaysand typically comprise an anode and a cathode between which issandwiched a multiplicity of thin layers including an electroluminescentlayer, electron injection and/or transport layer(s), hole injectionand/or transport layer(s) and optionally ancillary layers. The layersare typically built up by successive vacuum vapour depositionoperations, although it may be convenient to form one or more of thelayers e.g. the hole injection and hole transport layers by othermethods e.g. spin coating or ink jet printing.

A typical device comprises a transparent substrate on which aresuccessively formed an anode layer, a hole injector (buffer) layer, ahole transport layer, an electroluminescent layer, an electron transportlayer, an electron injection layer and an anode layer which may in turnbe laminated to a second transparent substrate. Top emitting OLED's arealso possible in which an aluminium or other metallic substrate carriesan ITO layer, a hole injection layer, a hole transport layer, anelectroluminescent layer, an electron transport layer, an electroninjection layer and an ITO or other transparent cathode, light beingemitted through the cathode. A further possibility is an inverted OLEDin which a cathode of aluminium or aluminium alloyed with a low workfunction metal carries successively an electron injection layer, anelectron transport layer, an electroluminescent layer, a hole transportlayer, a hole injection layer and an ITO or other transparent conductiveanode, emission of light being through the anode. If desired a holeblocking layer may be inserted e.g. between the electroluminescent layerand the electron transport layer. There may also be incorporated a layerof a reflectivity influencing material e.g. copper quinolate, vanadyloxyquinolate or vanadyl tetraphenoxy phthalocyanine e.g. as described inWO 2007/052083 (Kathirgamanathan et al.) the contents of which areincorporated herein by reference.

OLEDs of the invention include small molecule OLEDs, polymer lightemitting diodes (p-OLEDs), OLEDs that emit light by fluorescence, OLEDsthat emit light by phosphorescence (PHOLEDs) and OLEDs that emit lightby ion fluorescence (rare earth complexes) and include single-colour ormulti-colour active or passive matrix displays.

The front and/or rear plates of an OLED may be provided on front and/orrear surfaces with microlenses or microlens arrays e.g. an array ofmicrolenses of organic polymer (e.g. polymethyl methacrylate) printedonto an OLED substrate or plate e.g. a substrate or plate to form afront plate of an OLED, see e.g. Sun et al., Organic light emittingdevices with enhanced outcoupling via microlenses fabricated by imprintlithography, J. Appl. Phys. 100, 073106 (2006) and WO 2003/007663 (Moleret al., Princeton). Prismatic and lenticular films are available fromMicrosharp Corporation Limited of Watchfield, Oxfordshire and microlensand prismatic sheeting is available from 3M Corporation.

There may be employed conducting substrates: ITO/glass, transparentmetal coatings/glass, ATO, InZnO/glass and on plastics substrates.Conducting polymer coated plastics and glass may be used, for example,as anodes.

Anode

In many embodiments the anode is formed by a layer of tin oxide orindium tin oxide coated onto glass or other transparent substrate. Othermaterials that may be used include antimony tin oxide and indium zincoxide. As regards substrates, rigid or flexible transparent plasticsmaterials may be used, preferably materials which are dimensionallystable, impermeable to water (including water vapour) of relatively highTg. PEN is a preferred material, other materials that may be usedincluding PES, PEEK and PET. The plastics may be coated with aconductive film and may also have a barrier coating to improveresistance to moisture and hence improve service life.

Hole Injection Materials

A single layer may be provided between the anode and theelectroluminescent material, but in many embodiments there are at leasttwo layers one of which is a hole injection layer (buffer layer) and theother of which is a hole transport layer, the two layer structureoffering in some embodiments improved stability and device life (seeU.S. Pat. No. 4,720,432 (VanSlyke et al., Kodak). The hole injectionlayer may serve to improve the film formation properties of subsequentorganic layers and to facilitate the injection of holes into the holetransport layer.

Suitable materials for the hole injection layer which may be ofthickness e.g. 0.1-200 nm depending on material and cell type includehole-injecting porphyrinic compounds—see U.S. Pat. No. 4,356,429 (Tang,Eastman Kodak) e.g. zinc phthalocyanine copper phthalocyanine andZnTpTP, whose formula is set out below:

Particularly good device efficiencies, turn/on voltages and/or lifetimesmay be obtained where the hole injection layer is ZnTpTP both when thehost material for the electroluminescent layer is an organic complexe.g. a metal quinolate such as aluminium quinolate and when the hostmaterial is an organic small molecule material.

The hole injection layer may also be a fluorocarbon-based conductivepolymer formed by plasma polymerization of a fluorocarbon gas—see U.S.Pat. No. 6,208,075 (Hung et al; Eastman Kodak), a triarylaminepolymer—see EP-A-0891121 (Inoue et al., TDK Corporation) or aphenylenediamine derivative—see EP-A-1029909 (Kawamura et al.,Idemitsu).

Hole-Transport Materials

Hole transport layers which may be used are preferably of thickness 20to 200 nm.

One class of hole transport materials comprises polymeric materials thatmay be deposited as a layer by means of spin coating. Such polymerichole-transporting materials include poly(N-vinylcarbazole) (PVK),polythiophenes, polypyrrole, and polyaniline. Other hole transportingmaterials are conjugated polymers e.g. poly(p-phenylenevinylene) (PPV)and copolymers including PPV. Other preferred polymers are: poly(2,5dialkoxyphenylene vinylenes e.g.poly(2-methoxy-5-(2-methoxypentyloxy-1,4-phenylene vinylene),poly(2-methoxypentyloxy)-1,4-phenylenevinylene),poly(2-methoxy-5-(2-dodecyloxy-1,4-phenylenevinylene) and other poly(2,5dialkoxyphenylenevinylenes) with at least one of the alkoxy groups beinga long chain solubilising alkoxy group; polyfluorenes andoligofluorenes; polyphenylenes and oligophenylenes; polyanthracenes andoligoanthracenes; and polythiophenes and oligothiophenes.

A further class of hole transport materials comprises sublimable smallmolecules. For example, aromatic tertiary amines provide a class ofpreferred hole-transport materials, e.g. aromatic tertiary aminesincluding at least two aromatic tertiary amine moieties (e.g. thosebased on biphenyl diamine or of a “starburst” configuration), of whichthe following are representative:

It further includes spiro-linked molecules which are aromatic aminese.g. spiro-TAD(2,2′,7,7′-tetrakis-(diphenylamino)-spiro-9,9′-bifluorene).

A further class of small molecule hole transport materials is disclosedin WO 2006/061594 (Kathirgamanathan et al) and is based on diaminodianthracenes.

Typical compounds include:

-   9-(10-(N-(naphthalen-1-yl)-N-phenylamino)anthracen-9-yl)-N-(naphthalen-1-yl)-N-phenylanthracen-10-amine;-   9-(10-(N-biphenyl-N-2-n-tolylamino)anthracen-9-yl)-N-biphenyl-N-2-m-tolylamino-anthracen-10-amine;    and-   9-(10-(N-phenyl-N-m-tolylamino)anthracen-9-yl)-N-phenyl-N-m-tolylanthracen-10-amine.    Electroluminescent Materials

In principle any electroluminescent material may be used, includingmolecular solids which may be fluorescent dyes e.g. perylene dyes, metalcomplexes e.g. Alq₃, Ir(III)L₃, rare earth chelates e.g. Tb(II)complexes, dendrimers and oligomers e.g. sexithiophene, or polymericemissive materials. The electroluminescent layer may comprise asluminescent material a metal quinolate, iridium, ruthenium, osmium,rhodium, iridium, palladium or platinum complex, a boron complex or arare earth complex

One preferred class of electroluminescent materials comprises hostmaterials doped with dyes which may be fluorescent, phosphorescent orion-phosphorescent (rare earth). The term “electroluminescent device”includes electrophosphorescent devices.

Preferably the host is doped with a minor amount of a fluorescentmaterial as a dopant, preferably in an amount of 0.01 to 25% by weightof the doped mixture. As discussed in U.S. Pat. No. 4,769,292 (Tang etal., Kodak), the contents of which are included by reference, thepresence of the fluorescent material permits a choice from amongst awide latitude of wavelengths of light emission. In particular, asdisclosed in U.S. Pat. No. 4,769,292 by blending with the organometallic complex a minor amount of a fluorescent material capable ofemitting light in response to hole-electron recombination, the hue ofthe light emitted from the luminescent zone, can be modified. In theory,if a host material and a fluorescent material could be found forblending which have exactly the same affinity for hole-electronrecombination, each material should emit light upon injection of holesand electrons in the luminescent zone. The perceived hue of lightemission would be the visual integration of both emissions. However,since imposing such a balance of host material and fluorescent materialsis limiting, it is preferred to choose the fluorescent material so thatit provides the favoured sites for light emission. When only a smallproportion of fluorescent material providing favoured sites or lightemission is present, peak intensity wavelength emissions typical of thehost material can be entirely eliminated in favour of a new peakintensity wavelength emission attributable to the fluorescent material.

While the minimum proportion of fluorescent material sufficient toachieve this effect varies, in no instance is it necessary to employmore than about 10 mole percent fluorescent material, based of hostmaterial and seldom is it necessary to employ more than 1 mole percentof the fluorescent material. On the other hand, limiting the fluorescentmaterial present to extremely small amounts, typically less than about10⁻³ mole percent, based on the host material, can result in retainingemission at wavelengths characteristic of the host material. Thus, bychoosing the proportion of a fluorescent material capable of providingfavoured sites for light emission, either a full or partial shifting ofemission wavelengths can be realized. This allows the spectral emissionsof the EL devices to be selected and balanced to suit the application tobe served. In the case of fluorescent dyes, typical amounts are 0.01 to5 wt %, for example 2-3 wt %. In the case of phosphorescent dyes typicalamounts are 0.1 to 15 wt %. In the case of ion phosphorescent materialstypical amounts are 0.01-25 wt % or up to 100 wt %.

Choosing fluorescent materials capable of providing favoured sites forlight emission, necessarily involves relating the properties of thefluorescent material to those of the host material. The host can beviewed as a collector for injected holes and electrons with thefluorescent material providing the molecular sites for light emission.One important relationship for choosing a fluorescent material capableof modifying the hue of light emission when present in the host is acomparison of the reduction potentials of the two materials. Thefluorescent materials demonstrated to shift the wavelength of lightemission have exhibited a less negative reduction potential than that ofthe host. Reduction potentials, measured in electron volts, have beenwidely reported in the literature along with varied techniques for theirmeasurement. Since it is a comparison of reduction potentials ratherthan their absolute values which is desired, it is apparent that anyaccepted technique for reduction potential measurement can be employed,provided both the fluorescent and host reduction potentials aresimilarly measured. A preferred oxidation and reduction potentialmeasurement techniques is reported by R. J. Cox, PhotographicSensitivity, Academic Press, 1973, Chapter 15.

A second important relationship for choosing a fluorescent materialcapable of modifying the hue of light emission when present in the hostis a comparison of the band-gap potentials of the two materials. Thefluorescent materials demonstrated to shift the wavelength of lightemission have exhibited a lower band gap potential than that of thehost. The band gap potential of a molecule is taken as the potentialdifference in electron volts (eV) separating its ground state and firstsinglet state. Band gap potentials and techniques for their measurementhave been widely reported in the literature. The band gap potentialsherein reported are those measured in electron volts (eV) at anabsorption wavelength which is bathochromic to the absorption peak andof a magnitude one tenth that of the magnitude of the absorption peak.Since it is a comparison of band gap potentials rather than theirabsolute values which is desired, it is apparent that any acceptedtechnique for band gap measurement can be employed, provided both thefluorescent and host band gaps are similarly measured. One illustrativemeasurement technique is disclosed by F. Gutman and L. E. Lyons, OrganicSemiconductors, Wiley, 1967, Chapter 5.

With host materials which are themselves capable of emitting light inthe absence of the fluorescent material, it has been observed thatsuppression of light emission at the wavelengths of emissioncharacteristics of the host alone and enhancement of emission atwavelengths characteristic of the fluorescent material occurs whenspectral coupling of the host and fluorescent material is achieved. By“spectral coupling” it is meant that an overlap exists between thewavelengths of emission characteristic of the host alone and thewavelengths of light absorption of the fluorescent material in theabsence of the host. Optimal spectral coupling occurs when the emissionwavelength of the host is within ±25 nm of the maximum absorption of thefluorescent material alone. In practice advantageous spectral couplingcan occur with peak emission and absorption wavelengths differing by upto 100 nm or more, depending on the width of the peaks and theirhypsochromic and bathochromic slopes. Where less than optimum spectralcoupling between the host and fluorescent materials is contemplated, abathochromic as compared to a hypsochromic displacement of thefluorescent material produces more efficient results.

Useful fluorescent materials are those capable of being blended with thehost and fabricated into thin films satisfying the thickness rangesdescribed above forming the luminescent zones of the EL devices of thisinvention. While crystalline organometallic complexes do not lendthemselves to thin film formation, the limited amounts of fluorescentmaterials present in the host permit the use of fluorescent materialswhich are alone incapable of thin film formation. Preferred fluorescentmaterials are those which form a common phase with the host. Fluorescentdyes constitute a preferred class of fluorescent materials, since dyeslend themselves to molecular level distribution in the host. Althoughany convenient technique for dispersing the fluorescent dyes in the hostcan be used preferred fluorescent dyes are those which can be vacuumvapour deposited along with the host materials.

One class of host materials comprises metal complexes e.g. metalquinolates such as lithium quinolate, aluminium quinolate, titaniumquinolate, zirconium quinolate or hafnium quinolate which may be dopedwith fluorescent materials or dyes as disclosed in patent application WO2004/058913.

In the case of quinolates e.g. aluminium quinolate:

(a) the compounds below, for example, can serve as red dopants:

(b) the compounds below, for example can serve as green dopants:

wherein R is C₁-C₄ allyl, monocyclic aryl, bicycic aryl, monocyclicheteroaryl, bicyclic heteroaryl, aralkyl or thienyl, preferably phenyl;and

(c) for biphenyloxy aluminium bis-quinolate (BAlQ₂) or aluminiumquinolate the compounds perylene and9-(10-(N-(naphthalen-8-yl)-N-phenylamino)anthracen-9-yl)-N-(naphthalen-8-yl)-N-phenylanthracen-10-aminecan serve as a blue dopants.

Another preferred class of hosts is small molecules incorporatingconjugated aromatic systems with e.g. 4-10 aryl or heteroaryl ringswhich may bear substituents e.g. alkyl (especially methyl), alkoxy andfluoro and which may also be doped with fluorescent materials or dyes.

An example of a system of the above kind is a blue-emitting materialbased on the following compound (Compound H) as host

and perylene or9-(10-(N-(naphthalen-8-yl)-N-phenylamino)anthracen-9-yl)-N-(naphthalen-8-yl)-N-phenylanthracen-10-amineas dopant. Further examples of host materials which are small aromaticmolecules are shown below:

2,9-Bis(2-thiophen-2-yl-vinyl)-[1,10]phenanthroline may, as explainedabove, may be used as host in the electroluminescent layer or may bepresent on its own.

Blue-emitting materials may be based on an organic host (e.g. aconjugated aromatic compound as indicated above) and diarylamineanthracene compounds disclosed in WO 2006/090098 (Kathirgamanathan etal.) as dopants. For example, CBP may be doped with blue-emittingsubstituted anthracenes inter alia

-   9,10-bis(-4-methylbenzyl)-anthracene,-   9,10-bis-(2,4-dimethylbenzyl)-anthracene,-   9,10-bis-(2,5-dimethylbenzyl)-anthracene,-   1,4-bis-(2,3,5,6-tetramethylbenzyl)-anthracene,-   9,10-bis-(4-methoxybenzyl)-anthracene,-   9,10-bis-(9H-fluoren-9-yl)-anthracene,-   2,6-di-t-butylanthracene,-   2,6-di-t-butyl-9,10-bis-(2,5-dimethylbenzyl)-anthracene,-   2,6-di-t-butyl-9,10-bis-(naphthalene-1-ylmethyl)-anthracene.

Further blue-emitting materials may employ TCTA as host and it may bedoped with the blue phosphorescent materials set out below, see WO2005/080526 (Kathirgamanathan et al.):

Blue Phosphorescent Materials

Examples of green phosphorescent materials that may be employed with CBPor TAZ are set out below (see WO 2005/080526):

Green Phosphorescent Materials

Examples of red phosphorescent materials that may be employed with CBPor TAZ are set out below (see WO 2005/080526):

Red Phosphorescent Materials

As further dopants, fluorescent laser dyes are recognized to beparticularly useful fluorescent materials for use in the organic ELdevices of this invention. Dopants which can be used includediphenylacridine, coumarins, perylene and their derivatives. Usefulfluorescent dopants are disclosed in U.S. Pat. No. 4,769,292. One classof preferred dopants is coumarins. The following are illustrativefluorescent coumarin dyes known to be useful as laser dyes:

-   FD-1 7-Diethylamino-4-methylcoumarin,-   FD-2 4,6-Dimethyl-7-ethylaminocoumarin,-   FD-3 4-Methylumbelliferone,-   FD-4 3-(2′-Benzothiazolyl)-7-diethylaminocoumarin,-   FD-5 3-(2′-Benzimidazolyl)-7-N,N-diethylaminocoumarin,-   FD-6 7-Amino-3-phenylcoumarin,-   FD-7 3-(2′-N-Methylbenzimidazolyl)-7-N,N-diethylaminocoumarin,-   FD-8 7-Diethylamino-4-trifluoromethylcoumarin,-   FD-9    2,3,5,6-1H,4H-Tetrahydro-8-methylquinolazino[9,9a,1-gh]coumarin,-   FD-10 Cyclopenta[c]julolindino[9,10-3]-11H-pyran-11-one,-   FD-11 7-Amino-4-methylcoumarin,-   FD-12 7-Dimethylaminocyclopenta[c]coumarin,-   FD-13 7-Amino-4-trifluoromethylcoumarin,-   FD-14 7-Dimethylamino-4-trifluoromethylcoumarin,-   FD-15 1,2,4,5,3H,6H,    10H-Tetrahydro-8-trifluoromethyl[1]benzopyrano[9,9a,1-gh]quinolizin-10-one,-   FD-16 4-Methyl-7-(sulfomethylamino)coumarin sodium salt,-   FD-17 7-Ethylamino-6-methyl-4-trifluoromethylcoumarin,-   FD-18 7-Dimethylamino-4-methylcoumarin,-   FD-19 1,2,4,5,3H,6H,    100H-Tetrahydro-carbethoxy[1]benzopyrano[9,9a,1-gh]quinolizino-10-one,-   FD-20    9-Acetyl-1,2,4,5,3H,6H,10H-tetrahydro[1]benzopyrano[9,9a,1-gh]quinolizino-10-one,-   FD-21    9-Cyano-1,2,4,5,3H,6H,10H-tetrahydro[1]benzopyrano[9,9a,1-gh]quinolizino-10-one,-   FD22 9-(t-Butoxycarbonyl)-1,2,4,5,3,6H,6H,    10H-tetrahydro[1]-benzopyrano-[9,9a,1-gh]quinolizino-10-one,-   FD-23 4-Methylpiperidino[3,2-g]coumarin,-   FD-24 4-Trifluoromethylpiperidino[3,2-g]coumarin,-   FD-25 9-Carboxy-1,2,4,5,3H,6H,    OH-tetrahydro[1]benzopyrano[9,9a,1-gh]quinolizino-10-one,-   FD-26 N-Ethyl-4-trifluoromethylpiperidino[3,2-g].

Other dopants include salts of bis benzene sulphonic acid (requiredeposition by spin-coating rather than sublimation) such as

and perylene and perylene derivatives and dopants. Other dopants aredyes such as the fluorescent 4-dicyanomethylene-4H-pyrans and4-dicyanomethylene-4H-thiopyrans, e.g. the fluorescentdicyanomethylenepyran and thiopyran dyes. Useful fluorescent dyes canalso be selected from among known polymethine dyes, which include thecyanines, complex cyanines and merocyanines (i.e. tri-, tetra- andpoly-nuclear cyanines and merocyanines), oxonols, hemioxonols, styryls,merostyryls, and streptocyanines. The cyanine dyes include, joined by amethine linkage, two basic heterocyclic nuclei, such as azolium orazinium nuclei, for example, those derived from pyridinium, quinoliniun,isoquinolinium, oxazolium, thiazolium, selenazolium, indazolium,pyrazolium, pyrrolium, indolium, 3H-indolium, imidazolium, oxadiazolium,thiadioxazolium, benzoxazolium, benzothiazolium, benzoselenazolium,benzotellurazolium, benzimidazolium, 3H- or 1H-benzoindolium,naphthoxazolium, naphthothiazolium, naphthoselenazolium,naphthotellurazolium, carbazolium, pyrrolopyridinium,phenanthrothiazolium, and acenaphthothiazolium quaternary salts. Otheruseful classes of fluorescent dyes are 4-oxo-4H-benz-[d,e]anthracenesand pyrylium, thiapyrylium, selenapyrylium, and telluropyrylium dyes.

Further blue-emitting materials are disclosed in the following patents,applications and publications, the contents of which are incorporatedherein by reference:

-   U.S. Pat. No. 5,141,671 (Bryan, Kodak)—Aluminium chelates containing    a phenolato ligand and two 8-quinolinolato ligands.-   WO 00/32717 (Kathirgamanathan)—Lithium quinolate which is vacuum    depositable, and other substituted quinolates of lithium where the    substituents may be the same or different in the 2, 3, 4, 5, 6 and 7    positions and are selected from alky, alkoxy, aryl, aryloxy,    sulphonic acids, esters, carboxylic acids, amino and amido groups or    are aromatic, polycyclic or heterocyclic groups.-   US 2006/0003089 (Kathirgamanathan)—Lithium quinolate made by    reacting a lithium alkyl or alkoxide with 8-hydroxyquinoline in    acetonitrile.-   Misra, http://www.ursi.org/Proceedings/ProcGA05/pdf/D0.45(01720).pdf    Blue organic electroluminescent material bis-(2-methyl    8-quinolinolato) (triphenyl siloxy)aluminium (III) vacuum    depositable at 1×10⁻⁵ Torr.-   WO 03/006573 (Kathirgamanathan et al)—Metal pyrazolones.-   WO 2004/084325 (Kathirgamanathan et al)—Boron complexes.-   WO 2005/080526 (Kathitgamanathan et al)—Blue phosphorescent    iridium-based complexes.-   Ma et al., Chem. Comm. 1998, 2491-2492 Preparation and crystal    structure of a tetranuclear zinc(II) compound [Zn₄O(AID)₆] with    7-azaindolate as a bridging ligand. Fabrication of inter alia a    single-layer LED by vacuum deposition of this compound (<200° C.,    2×10⁻⁶ Torr) onto a glass substrate coated with indium-tin oxide to    form a thin homogeneous film was reported.

Further electroluminescent materials which can be used include metalquinolates such as aluminium quinolate, lithium quinolate, titaniumquinolate, zirconium quinolate, hafnium quinolate etc.

Many further electroluminescent materials that may be used are disclosedin WO 2004/050793 (pyrazolones), WO 2004/058783 (diiridium metalcomplexes), WO 2006/016193 (dibenzothiophenyl metal complexes) and WO2006/024878 (thianthrene metal complexes), see also WO 2006/040593 thecontents of which are incorporated herein by reference. Rare earthchelates, in particular may be employed as green and red emitters.Furthermore, there may be used as electroluminescent materialsconducting polymers e.g. polyaniline, phenylene vinylene polymers,fluorene homopolymers and copolymers, phenylene polymers, as indicatedbelow:

Conducting Polymers

Electron Transport Material

Known electron transport materials may be used, including, for example,quinolates.

Aluminium quinolate is thermally and morphologically stable to beevaporated into thin films, easily synthesized and purified and iswidely used despite its problems of relatively low mobility, bandgap andtendency to ashing during sublimation. As disclosed in patentapplication GB 0625541.8 filed 22 Dec. 2006, imporived electrontransport materials consist of or comprise zirconium or hafniumquinolate, zirconium quinolate being preferred for many embodiments.

Zirconium quinolate has a particularly advantageous combination ofproperties for use as an electron transport material and which identifyit as being a significant improvement on aluminium quinolate for use asan electron transport material. It has high electron mobility. Itsmelting point (388° C.) is lower than that of aluminium quinolate (414°C.). It can be purified by sublimation and unlike aluminium quinolate itresublimes without residue, so that it is even easier to use thanaluminium quinolate. Its lowest unoccupied molecular orbital (LUMO) isat −2.9 eV and its highest occupied molecular orbital (HOMO) is at −5.6eV, similar to the values of aluminium quinolate. Furthermore,unexpectedly, it has been found that when incorporated into a chargetransport layer it slows loss of luminance of an OLED device at a givencurrent with increase of the time for which the device has beenoperative (i.e. increases device lifetime), or increases the lightoutput for a given applied voltage, the current efficiency for a givenluminance and/or the power efficiency for a given luminance. Embodimentsof cells in which the electron transport material is zirconium quinolatecan exhibit reduced turn-on voltage and up to four times the lifetime ofsimilar cells in which the electron transport material is zirconiumquinolate. It is compatible with aluminium quinolate when aluminiumquinolate is used as host in the electroluminescent layer of an OLED,and can therefore be employed by many OLED manufacturers with only smallchanges to their technology and equipment. It also forms a goodelectrical and mechanical interface with inorganic electron injectionlayers e.g. a LiF layer where there is a low likelihood of failure bydelamination. Of course zirconium quinolate can be used both as host inthe electroluminescent layer and as electron transfer layer. Theproperties of hafnium quinolate are generally similar to those ofzirconium quinolate.

Zirconium or hafnium quinolate may be the totality, or substantially thetotality of the electron transport layer. It may be a mixture ofco-deposited materials which is predominantly zirconium quinolate. Thezirconium or hafnium may be doped as described in GB 06 14847.2 filed 26Jul. 2006, the contents of which are incorporated herein by reference.Suitable dopants include fluorescent or phosphorescent dyes or ionfluorescent materials e.g. as described above in relation to theelectroluminescent layer, e.g. in amounts of 0.01-25 wt % based on theweight of the doped mixture. Other dopants include metals which canprovide high brightness at low voltage. Additionally or alternatively,the zirconium or hafnium quinolate may be used in admixture with anotherelectron transport material. Such materials may include complexes ofmetals in the trivalent or pentavalent state which should furtherincrease electron mobility and hence conductivity. The zirconium andhafnium quinolate may be mixed with a quinolate of a metal of group 1,2, 3, 13 or 14 of the periodic table, e.g. lithium quinolate or zincquinolate. Preferably the zirconium or hafnium quinolate comprises atleast 30 wt % of the electron transport layer, more preferably at least50 wt %.

Electron Injection Material

The electron injection layer is deposited direct onto the cathode andcomprises a Schiff base of one of the above mentioned formulae which maybe used alone or in combination with another electron injection materiale.g. a quinolate such as lithium or zirconium quinolate. The Schiff basepreferably comprises at least 30 wt % of the electron injection layer,more preferably at least 50 wt %.

In the formula set out above, R₁ may be polycyclic aryl e.g. naphthyl,anthracenyl, tetracenyl, pentacenyl or a perylene or pyrene compound ormay have up to 5 aromatic rings arranged in a chain e.g. biphenyl. It ispreferably phenyl or substituted phenyl. R₂ and R₃ together may form thesame groups as R₁ and are preferably phenyl or substituted phenyl. Wheresubstituents are present they may be methyl, ethyl, propyl or butyl,including t-butyl substituted, or may be methoxy, ethoxy, propoxy orbutoxy including t-butoxy substituted. Particular compounds include

A preferred group of compounds is of the formula

wherein R₁ is phenyl or phenyl substituted with one or more C₁-C₄ alkylgroups e.g. methyl groups and R₂ and R₃ together form phenyl or phenylsubstituted by one or more C₁-C₄ alkyl groups e.g, methyl groups. Thesecompounds which have N—Li—O in a 6-membered ring have been found to haverelatively low vacuum sublimation temperatures, especially when thereare methyl substituents. Similar behaviour is expected of compounds offormula:

wherein, as previously, R₁ is phenyl or phenyl substituted with one ormore C₁-C₄ alkyl groups e.g. methyl groups and R₂ and R₃ together formphenyl or phenyl substituted by one or more C₁-C₄ alkyl groups e.g.methyl groupsCathode

In many embodiments, aluminium is used as the cathode either on its ownor alloyed with elements such as magnesium or silver, although in someembodiments other cathode materials e.g. calcium may be employed. In anembodiment the cathode may comprise a first layer of alloy e.g. Li—Ag,Mg—Ag or Al—Mg closer to the electron injection or electron transportlayer and an second layer of pure aluminium further from the electroninjection or electron transport layer. Cathode materials may also be ontransparent plate materials which may be of glass or may be of plasticswhich may be rigid or flexible and may be optically transparent. Asregards plastics substrates, rigid or flexible transparent plasticsmaterials may be used, preferably materials which are dimensionallystable, impermeable to water (including water vapour) of relatively highTg. PEN is a preferred material, other materials that may be usedincluding PES, PEEK and PET. The plastics may be coated with aconductive film and may also have a barrier coating to improveresistance to moisture which may be encountered under working conditionse.g. atmospheric moisture.

How the invention may be put into effect will now be described withreference to the following examples.

Preparative Methods Zirconium tetrakis(8-hydroxyquinolate) (Zrq₄)

To a solution of 8-Hydroxyquinoline (20.0 g, 138 mmol) in ethanol (300mL, 95%) was added zirconium (IV) chloride (8.03 g, 34 mmol) in ethanol(50 mL). The pH of the solution was increased by dropwise addition ofpiperidine (total ˜15 mL, 150 mmol) until a yellow precipitate forms.The suspension was heated to approx. 60° for 1 hour, cooled to roomtemperature and the precipitate collected onto a Buchner funnel. Thiswas thoroughly washed with ethanol (3×100 mL, 95%) and dried undervacuum. Initial purification was performed by Soxhlet extraction with1,4-dioxane for 24 hours. Concentration of the 1,4-dioxane yields ayellow precipitate, which was collected on a Buchner funnel and washedwith ethanol (100 mL, 95%). This sample was dried in a vacuum oven at80° C. for 4 hours. Final purification was achieved by sublimation.Yield—75% before sublimation. (60% after 2 sublimations). Sublimation(390° C., 10-Torr), m.p. 383° C.

Hafnium tetrakis(8-hydroxyquinolate) (Hfq₄)

To a solution of 8-Hydroxyquinoline (5.44 g, 37.5 mmol) in ethanol (200mL, 95%) was added hafnium (IV) chloride (3.0 g, 9.37 mmol) in ethanol(100 mL), followed by a further 300 mL water. The pH of the solution wasincreased by dropwise addition of piperidine until a yellow precipitateforms. The resulting yellow precipitate was collected and washed withethanol (100 mL, 95%), water (200 mL) and finally ethanol (100 mL, 95%).The sample was dried under vacuum at 80° C. until no further weight losswas detected. Sublimation (400° C., 10⁻⁶ Torr) yielded an analyticalsample (4.5 g, 64%), m.p. 398° C.

1a Synthesis of N-salicylideneaniline

To a mixture of salicylaldehyde (40 mL, 45.84 g, 375.37 mmol)) andaniline (22 mL, 31.72 g, 374.66 mmol), was added ethanol (90 mL), 8drops of concentrated hydrochloric acid and water (10 mL). This reactionmixture was refluxed for one hour, allowed to cool to room temperatureand left in the refrigerator over the weekend. A large quantity oforange solid was formed after 2 hours in the refrigerator. It wasfiltered off and washed with ethanol. Recrystallisation from ethanolafforded 31.53 g of product.

1b Synthesis of Lithium 2-phenyliminomethyl phenolate (Compound A)

Lithium isopropoxide (300 mL, 20.7 g, 66.03 mmol) was added slowly to asolution of N-salicylideneaniline (61.83 g, 66.03 mmol) in driedacetonitrile (200 mL) under nitrogen atmosphere. A pale yellowprecipitate was formed and was left stirring overnight. It was filteredoff, washed thoroughly with acetonitrile and dried in vacuum oven for 8hours at 80° C. Giving 61.6 g of product (97% yield). Sublimation (260°C., 10⁻⁶ Torr.) yielded an analytical sample (25.1 g from 29.2 g).

2a Synthesis of N-salicylidene-2-methylaniline,N-salicylidene-3-methylaniline and N-salicylidene-4-methylaniline

To a mixture of salicylaldehyde (15.00 mL, 17.16 g, 140.76 mmol) ando-toluidine (15.00 mL, 15.06 g, 140.54 mmol), was added ethanol (30 mL),8 drops of concentrated hydrochloric acid and water (10 mL). Thisreaction mixture was refluxed for two hours and left vigorously stirredover the weekend at room temperature. The yellow crystalline solid wascollected by filtration, recrystallised from ethanol, washed thoroughlywith ethanol and dried in vacuum oven for over 8 hours at >40° C.,giving 17.24 g of product (58% yield). It gave a bright yellowfluorescence.

N-salicylidene-3-methylaniline and N-salicylidene-4-methylaniline weresynthesised using the procedure described above, starting withm-toluidine and p-toluidine, respectively.

2b Synthesis of N-salicylidene-2-methylaniline lithium complex and thecorresponding 3-methylaniline and 4-methylaniline complexes

To a stirred solution of N-salicylidene-2-methylaniline (11.00 g, 52.07mmol) in dry acetonitrile (80 mL), was added lithium isopropoxide (52.00mL, 3.58 g, 54.34 mmol). A small quantity of white precipitate wasslowly formed after 15 minutes of stirring. This reaction mixture wasleft vigorously stirred overnight at room temperature. The white solidwas collected by filtration, washed thoroughly with acetonitrile anddried in vacuum oven for over 8 hours at 80° C. Giving 10.47 g ofproduct (93% yield). Sublimation (235° C., 10⁻⁶ Torr.) yielded ananalytical sample (6.5 g from 10.2 g).

The corresponding 3-methyl and 4-methyl compounds were synthesised usingthe same procedure as described above and usingN-salicylidene-3-methylaniline and N-salicylidene-4-methylanilinerespectively as starting materials.

3a Synthesis of N-salicylidene-2,3-dimethylaniline,N-salicylidene-2,4-dimethylaniline andN-salicylidene-2,5-dimethylaniline

To a mixture of salicylaldehyde (15.00 mL, 17.16 g, 140.76 mmol) and2,3-dimethylaniline (17.20 mL, 17.08 g, 140.94 mmol), was added ethanol(30 mL), 8 drops of concentrated hydrochloric acid and water (10 mL).This reaction mixture was refluxed for two hours and left vigorouslystirred overnight at room temperature. The resulting yellow solid wascollected by filtration, recrystallised from ethanol, washed thoroughlywith ethanol and dried in vacuum oven for over 8 hours at >40° C.,giving 29.98 g of product (95% yield).

N-salicylidene-2,4-dimethylaniline andN-salicylidene-2,5-dimethylaniline were synthesised using the sameprocedure and using 2,4-dimethylaniline and 2,5-dimethylanilinerespectively as starting materials.

3b Synthesis of N-salicylidene-2,3-dimethylaniline lithium complex,N-salicylidene-2,4-dimethylaniline lithium complex andN-salicylidene-2,5-dimethylaniline lithium complex

To a stirred solution of N-salicylidene-2,3-dimethylaniline (6.00 g,26.63 mmol) in dry acetonitrile (30 mL), was added lithium isopropoxide(25.50 mL, 1.76 g, 26.65 mmol). A yellow precipitate was immediatelyformed, and the reaction mixture was left vigorously stirred overnightat room temperature. The yellow solid was collected by filtration,washed thoroughly with acetonitrile and dried in vacuum oven for over 8hours at 80° C., giving 4.94 g of product (80% yield). Sublimation (250°C., 10⁻⁶ Torr.) yielded an analytical sample (2.2 g from 3.0 g).

The corresponding 2,4-dimethyl and 2,5-dimethyl complexes weresynthesised using the same procedure described and usingN-salicylidene-2,4-dimethylaniline andN-salicylidene-2,5-dimethylaniline respectively as starting materials.

4a Synthesis of N-salicylidene-2-cyanoaniline,N-salicylidene-3-cyanoaniline and N-salicylidene-4-cyanoaniline

To a mixture of salicylaldehyde (13.5 mL, 15.47 g, 126.61 mmol) andanthranitonitrile (15.00 g, 126.97 mmol), was added ethanol (30 mL), 8drops of concentrated hydrochloric acid and water (10 mL). This reactionmixture was refluxed for two hours and left stirring at room temperatureovernight. The yellow precipitate was filtered, washed with ethanol. Theyellow solid was recrystallised from ethanol and left drying in thevacuum oven, giving 19.49 g of product (69% yield).

N-salicylidene-3-cyanoaniline and N-salicylidene-4-cyanoaniline weresynthesised using the same procedure and using 3-aminobenzonitrile and4-aminobenzonitrile respectively as starting materials.

4b Synthesis of N-salicylidene-2-cyanoanilinelithium complex,N-salicylidene-3-cyanoanilinelithium complex andN-salicylidene-4-cyanoanilinelithium complex

Lithium isopropoxide (44 mL, 3.04 g, 45.98 mmol) was added slowly to asolution of N-salicylidene-2-cyanoaniline (10.00 g, 45.00 mmol) in driedacetonitrile (40 mL) under nitrogen atmosphere. A yellow precipitate wasslowly formed and was vigorously stirred for overnight. The yellowprecipitate was filtered off, washed thoroughly with acetonitrile anddried in vacuum oven at 80° C. for 8 hours, giving 10.40 g of product(100% yield).

The corresponding 3-cyano and 4-cyano complexes were synthesised usingthe same procedure and using N-salicylidene-3-cyanoaniline andN-salicylidene-4-cyanoaniline, respectively.

5a Synthesis of N-benzylidene-2-hydroxyaniline

To a solution of 2-aminophenol (28.55 g, 261.61 mmol) in ethanol (40mL), was added benzaldehyde (30 mL, 31.32 g, 295.14 mmol), and thisreaction mixture was refluxed for two hours, and left stirring at roomtemperature overnight. The creamy white precipitate was filtered, washedwith ethanol and dried in vacuum oven at 80° C. for 4 hours. Giving40.27 g (78% yields).

5b Synthesis of N-benzylidene-2-hydroxyanilinelithium complex

Lithium isopropoxide (50 mL, 3.45 g, 52.25 mmol) was added slowly to asolution of N-benzylicylidene-2-hydroxyanilne (10.25 g, 52.24 mmol) indried acetonitrile (40 mL) under nitrogen atmosphere. A yellowprecipitate was formed and was vigorously stirred for over the weekend.The yellow precipitate was filtered off, washed thoroughly withacetonitrile and dried in vacuum oven at 80° C. for 8 hours, giving10.55 g of product (100% yield).

6a Synthesis of N-naphthalidene-2-hydroxyaniline

To a stirred solution of 2-aminophenol (14.46 g, 132.54 mmol) in ethanol(30 mL), was added 1-naphthaldehyde (18 mL, 20.7 g, 132.54 mmol). Thisreaction mixture was refluxed for three hours, and left stirringovernight. The yellowish brown precipitate formed was filtered off,washed with ethanol and dried in vacuum oven at 60° C. for 8 hours,giving 24.34 g of product (74% yields).

6b Synthesis of N-naphthalidene-2-hydroxyanilinelithium complex

Lithium isopropoxide (20 mL, 1.38 g, 20.90 mmol) was added slowly to astirred solution of N-naphthalidene-2-hydroxyaniline (5.17 g, 20.91mmol) in dried acetonitrile (35 mL) under nitrogen atmosphere. Thisreaction mixture was left stirring overnight. The orange precipitate wascollected by filtration, giving 5.12 g of product (97% yield).

7 Synthesis of the N,N′disalicylidene-1,2-phenylenediamino-dilithiumcomplex

Lithium isopropoxide (50 mL, 3.45 g, 52.25 mmol) was added slowly to asolution of N,N′disalicylidene-1,2-phenylenediamine (8.26 g, 26.11 mmol)in dried acetonitrile (70 mL) under nitrogen atmosphere. A pale yellowprecipitate was formed and was left stirring for two hours. The yellowsolid was filtered off, washed thoroughly with acetonitrile and dried invacuum oven for 8 hours at 80° C., giving 2.4 g of product (71% yield).Sublimation (340° C., 10⁻⁶ Torr.) yielded an analytical sample (1.3 gfrom 2.3 g).

8a Synthesis of Schiff base of thiophene-2-carboxaldehyde with4-aminophenol[(E)-2-((thiophen-2-yl)methyleneamino)phenol)]

To a dry flask under nitrogen was charged 4-aminophenol (10.0 g, 92mmol) and ethanol (50 ml). To this was added, with stirring,thiophene-2-carboxaldehyde (11.3 g, 9.4 ml, 0.1 mol). The mixture washeated at reflux (90° C.) for 2.5 hours and subsequently allowed to cooldown. The solvent was then removed from the reaction mixture to give adark brown liquid which was left in a fridge for more than an hour andcharged with petroleum ether (40-60° C.). and allowed to solidify. Abrown solid was filtered and then slurred with ethanol-pet ether mixture(2:8). Finally, the solid was again filtered, washed with pet ether anddried in vacuo for 18 h. Yield of product (crude): 15.0 g (78%).

8b Synthesis of lithium complex of(E)-2-((thiophen-2-yl)methyleneamino)phenol

To a dry flask under nitrogen was charged Schiff base (4.0 g, 20 mmol)and dry acetonitrile (50 ml). To this was syringed slowly, withstirring, lithium isopropoxide (1.0M) (1.4 g, 22 ml, 22 mmol). Themixture was left overnight at room temperature while stirringvigorously. A yellow solid was filtered and then slurred withacetonitrile (30 ml). Finally, the solid was again filtered undersuction and dried in vacuo for 18 h. Yield of product (crude): 3.8 g(90%).

9a Synthesis of Schiff base of biphenyl-4-carboxaldehyde with4-aminophenol[2-(biphenyl-4-yl-methyleneamino)phenol]

To a dry flask under nitrogen was charged 4-aminophenol (5.0 g, 46 mmol)and ethanol (50 ml). To this was added, with stirring,biphenyl-4-carboaldehyde (8.4 g, 46 mmol). The mixture was heated atreflux (90° C.) for 2½ hours and subsequently allowed to cool down.Filtration gave a yellow solid, which was then slurred with ethanol-petether mixture (1:1). Finally, the solid was again filtered, washed withpet ether and dried in vacuo for 18 h. Yield of product (crude): 10.3 g(80%).

9b Synthesis of lithium complex of Schiff base of 2-aminothiazole withsalicyladehyde

To a dry flask under nitrogen was charged Schiff base (4.0 g, 20 mmol)and dry acetonitrile (50 ml). To this was syringed slowly, withstirring, lithium isopropoxide (1.0M) (1.4 g, 22 ml, 22 mmol). Themixture was left overnight at room temperature while stirringvigorously. A brown solid was filtered and then slurred withacetonitrile (30 ml). Finally, the solid was again filtered undersuction and dried in vacuo for 18 h. Yield of product (crude): 3.7 g(85%).

10a Synthesis of Schiff base of 2-aminothiazole with salicyladehyde(2-[(E)-(thiazol-2-ylimino)methyl)phenol]

To a dry flask under nitrogen was charged 2-aminothiazole (5.0 g, 50mmol) and ethanol (50 ml). To this was added, with stirring,salicyladehyde (6.1 g, 5.3 ml, 50 mol). The mixture was heated at reflux(90° C.) for 2.5 hours and subsequently allowed to cool down. Thesolvent then removed from the reaction mixture resulting in a dark brownliquid which was left in a fridge for more than an hour and charged withpetroleum ether (40-60° C.), after which a greenish-yellow solid formedand was filtered and then slurred with ethanol-pet ether mixture (2:8).Finally, the solid was again filtered, washed with pet ether and driedin vacuo for 18 h. Yield of product (crude): 6.0 g (60%).

10b Synthesis of lithium complex of Schiff base of 2-aminothiazole withsalicyladehyde

To a dry flask under nitrogen was charged Schiff base (4.0 g, 20 mmol)and dry acetonitrile (50 ml). To this was syringed slowly, withstirring, lithium isopropoxide (1.0M) (1.4 g, 22 ml, 22 mmol). Themixture was left overnight at room temperature while stirringvigorously. A brown solid was filtered and then slurred withacetonitrile (30 ml). Finally, the solid was again filtered undersuction and dried in vacuo for 18 h. Yield of product (crude): 3.7 g(85%).

Properties of lithium compounds synthesized as described above are givenin the accompanying Table. Vacuum sublimation temperatures of some ofthe compounds are shown in FIG. 13. It will be noted that all thecompounds tested exhibit vacuum sublimation temperatures <250° C.,whereas LiF exhibits a vacuum sublimation temperatures >550° C. The lowvacuum sublimation temperature exhibited by some of the compounds of theinvention coupled with their electron injection properties is ofsignificant advantage in device manufacture since thermal effects onalready deposited layers are reduced.

M. Pt. E.A. DSC UV-Vis. FL. (%) Peak Mass Tg λ_(max) (nm) λ_(max) (nm)Name Structure Theory Found (° C.) Spec (° C.) Solution SolutionComments Com- pound A

C = 76.85 H = 4.96 N = 6.89 C = 77.05 H = 4.90 N = 6.98 299 1) 203 2)406 3) 617 4) 820 No Tg 341 403 472 Vacuum Processable Com- pound B

C = 77.42 H = 5.57 N = 6.45 C = 77.46 H = 5.54 N = 6.55 252 1) 217 2)434 3) 651 No Tg 341 368 416 506 Vacuum Processable Com- pound C

C = 77.42 H = 5.57 N = 6.45 C = 76.88 H = 5.37 N = 6.34 247 To be deter-mined No Tg 341 366 421 497 Vacuum Processable Com- pound D

C = 77.42 H = 5.57 N = 6.45 C = 77.23 H = 5.50 N = 6.39 329 To be deter-mined No Tg 343 367 443 505 Vacuum Processable Com- pound E

C = 77.92 H = 6.10 N = 6.06 C = 78.03 H = 6.12 N = 6.18 283 To be deter-mined 111 340 363 429 500 Vacuum Processable Com- pound F

C = 77.92 H = 6.10 N = 6.06 C = 78.11 H = 6.08 N = 5.99 259 To be deter-mined 95 346 371 424 505 Vacuum Processable Com- pound G

C = 77.92 H = 6.10 N = 6.06 C = 78.08 H = 6.11 N = 6.14 245 To be deter-mined No Tg 342 366 423 507 Vacuum Processable Com- pound I

C = 73.69 H = 3.98 N = 12.28 C = 73.18 H = 3.81 N = 12.12 307 To bedeter- mined No Tg Not measured Not measured Solution Processable Com-pound J

C = 73.69 H = 3.98 N = 12.28 C = 72.65 H = 3.75 N = 11.98 290 To bedeter- mined 141 Not measured Not measured Solution Processable Com-pound K

C = 73.69 H = 3.98 N = 12.28 C = 68.39 H = 4.53 N = 11.07 147 To bedeter- mined No Tg Not measured Not measured Solution Processable Com-pound L

C = 76.85 H = 4.96 N = 6.89 C = 77.00 H = 5.29 N = 6.78 324 1) 210 2)413 3) 617 4) 820 No Tg 355 381 493 Vacuum & Solution Processable Com-pound M

C = 80.63 H = 4.78 N = 5.53 C = 80.07 H = 4.77 N = 5.45 285 1) 253 2)525 3) 787 156 364 394 Vacuum & Solution Processable Com- pound N

C = 57.14 H = 3.36 N = 13.33 C = 57.80 H = 3.07 N = 13.13 355 To bedeter- mined No Tg Not measured Not measured Solution Processable Com-pound O

C = 81.72 H = 5.05 N = 5.02 C = 80.08 H = 4.93 N = 4.88 389 To be deter-mined No Tg Not measured Not measured Solution Processable Com- pound P

C = 63.16 H = 3.85 N = 6.70 C = 59.39 H = 3.85 N = 6.21 336 To be deter-mined No Tg Not measured Not measured Solution Processable Com- pound Q

C = 73.19 H = 4.30 N = 8.54 C = 72.42 H = 4.10 N = 8.41 ~405 1) 328 2)656 3) 984 No Tg 387 462 Vacuum & Solution ProcessableDevice Structure

A pre-etched ITO coated glass piece (10×10 cm²) was used. The device wasfabricated by sequentially forming layers on the ITO, by vacuumevaporation using a Solciet Machine, ULVAC Ltd. Chigasaki, Japan. Theactive area of each pixel was 3 mm by 3 mm. The coated electrodes wereencapsulated in an inert atmosphere (nitrogen) with UV-curable adhesiveusing a glass back plate. Electroluminescence studies were performedwith the ITO electrode was always connected to the positive terminal.The current vs. voltage studies were carried out on a computercontrolled Keithly 2400 source meter.

EXAMPLE 1

Devices with green emitters were formed by the method described aboveconsisting of an anode layer, buffer layer, hole transport layer,electroluminescent layer (doped metal complex), electron transportlayer, electron injection layer and cathode layer, film thicknessesbeing in nm;

ITO/ZnTp TP (20)/α-NBP(50)/Alq₃:DPQA (40:0.1)/Zrq₄ (20)/EIL(0.5)/Al

wherein DPQA is diphenyl quinacridone and EIL is the electron injectionlayer and is LiF or is Compound A.

Compared to cells in which the EIL was LiF, that using Compound A showedgreater luminance, greater current and power efficiencies for a givenluminance and greater current density for a given applied voltage (FIGS.1-4). In green-emitting OLEDs compound A also gives better results thanlithium quinolate when used as an electron injection layer andevaporates below 300° C.

EXAMPLE 2

Further devices in which the electron injection layer was a compoundmade as described above were manufactured and evaluated in relation tosimilar devices made using Lif as the injection layer. Performanceresults are shown in FIGS. 5-12. Compounds H and Czl used in theelectroluminescent layer in some tests are shown below:

The invention claimed is:
 1. A compound of the formula

wherein R₁ is a 1-5 ring aryl, aralkyl or heteroaryl group which isoptionally substituted with one or more C₁-C₄ alkyl, alkoxy or cyano; R₂and R₃ together form a 1-5 ring aryl, aralkyl or heteroaryl group whichis optionally substituted with C₁-C₄ alkyl, alkoxy or cyano; R₄ ishydrogen, C₁-C₄ alkyl or aryl; and Ar is monocyclic, bicyclic ortricyclic aryl or heteroaryl which is optionally substituted with one ormore C₁-C₄-alkyl or alkoxy groups.
 2. The compound of claim 1, whereinR₁ is phenyl or substituted phenyl and R₂ and R₃ together form phenyl orsubstituted phenyl.
 3. The compound of claim 1, which is of formula

wherein R₁ is phenyl or phenyl substituted with one or more C₁-C₄ alkylgroups and R₂ and R₃ together form phenyl or phenyl substituted by oneor more C₁-C₄ alkyl groups, and R₄ is hydrogen, C₁-C₄ alkyl or aryl. 4.The compound of claim 3, wherein R₁ and/or R₂ and R₃ represent phenylsubstituted with one or more methyl groups.
 5. The compound of claim 1,wherein the compound is selected from the following:


6. The compound of claim 1, wherein the compound is